The present invention relates to the evaluation of the grip of a vehicle on a road. It relates more particularly to the determination of grip characteristics between the road and a vehicle wheel fitted with an elastic tire, such as an inflated pneumatic tire or a non-pneumatic elastic tire, which rolls on the road.
The present invention also relates to the various electronic assistance devices used, for example, for antilock control of the brakes of a vehicle or antiskid control of the drive wheels, control of the direction of a vehicle or other forms of control or monitoring, as for instance the tire pressures. It is known that such devices reconstruct the grip coefficient (μ) of the tires on the road by calculation, without having carried out any measurement either of the grip coefficient or of the forces developed in the contact of the tires with the ground. Even though these devices provide significant assistance and extra safety, their operation would benefit greatly from the use of a measured value, or a value estimated on the basis of real measurements carried out on the tire during operation.
For this reason, it is an object of the present invention to provide a way of evaluating the grip of a vehicle on a road, and more precisely of its wheels or tires or elastic tires, these terms being regarded as equivalent in the context of the present invention. It relates more particularly to the determination of grip characteristics between the road and a vehicle wheel fitted with an elastic tire, such as an inflated pneumatic tire or a non-pneumatic elastic tire, which rolls on the road.
The various electronic assistance devices mentioned above would therefore usefully benefit from “real-time” indications of the grip conditions liable to affect the handling of a vehicle, especially when it undergoes an acceleration due to a driving force or a braking force, or due to a change of direction of movement. The invention aims to provide a method of achieving this efficiently.
In what follows, “maximum grip potential” refers to the ratio between the maximum tangential force (transverse or longitudinal, or both combined) and the normal force which the wheel can experience. In the text, this is also denoted by the term “maximum grip coefficient”, or the letter μ.
“Overall forces” refers to the three components of forces Fx, Fy and Fz applied to the center of the wheel, and the self-alignment torque N about the Z axis.
With a view to estimating the maximum grip potential, it has been proposed that the tread of a tire, or certain specially adapted elements of the tread, be fitted with sensors intended to measure or estimate the forces generated locally, in particular under slip conditions. Although highly promising, these approaches nevertheless involve certain intrinsic difficulties. Indeed, it is difficult to ensure correct operation of a sensor in this zone of the tire, in particular when there is a wear of the tread, throughout the life of the tire. Furthermore, the estimates provided by these sensors are highly local and sensitive to the surface condition of the road.
Since the intention is actually to estimate the maximum grip potential of the wheel, this still remains to be determined on the basis of the measured local potential.
The invention described in detail here differs from these local approaches. It proposes to use a measurement of the overall deformations of the tire so as to obtain information about the maximum grip potential of the wheel on the ground. Indeed, when the tire is subjected to a constraint, the point of application of the forces being applied in the contact area depends, inter alia, on the maximum grip coefficient because, as soon as a part of the contact area of the wheel on the road is slipping, its contribution to the tangential forces is saturated at a level which depends on the grip coefficient. The deformations of the tire are themselves sensitive to the movement of this point of application. In particular, the circumferential extension of the sidewalls, which is sensitive to the applied forces, is also sensitive to the movement of the point of application of the forces in the contact area.
The proposed method uses measurements of the circumferential deformations of the sidewall at certain azimuths of the tire in order to allow estimation of the maximum grip coefficient.
In order to provide a good estimate of μ, the method requires that there be a slip zone in the contact area, which may be created either by a special design of the tire or by a sufficient level of constraint applied to the tire. In order to obtain reliable information even when there is little slipping, the invention proposes that the percentage of the potential used be estimated in addition to the maximum grip potential. The reason is that this quantity has the advantage of being easier to estimate as an absolute value, even for small constraints.
The method according to the invention for determining the grip coefficient μ in the contact area of a tire on a road includes the following steps:
selecting a plurality of fixed points in space, which lie at different azimuths along the circumference in at least one sidewall of the tire,
carrying out a corresponding number of measurements of circumferential distance variation (extension or contraction) at these fixed points when the tire is rolling on the road, and the measurement signals are processed so as to extract the said grip coefficient μ from them.
Advantageously, the said grip coefficient μ is derived from at least 5 measurements of circumferential distance variation (extension or contraction) carried out in at least one sidewall of the tire, at 5 fixed points in space (that is to say ones that are fixed in the reference frame associated with the vehicle) which lie at different azimuths along the circumference.
The method of the invention is based on recognition of the fact that the forces acting between the tread of the tire and the road, as well as their distribution in the contact area, cause a substantial and reproducible deformation in the form of a circumferential extension or contraction of the sidewalls of the tires. This circumferential extension or contraction, if it is possible to measure it individually during rotation of the tire in real time, can make it possible to know at each instant the direction and magnitude of the forces acting on the tire, as well as the sign and the magnitude of the self-alignment torque exerted by the tire and the grip coefficient of the tire on the road.
According to one particular aspect, the invention proposes to estimate the circumferential contraction or extension of the sidewalls by measuring the distance between the threads of the carcass ply in the sidewalls. It is also possible to measure the distance between wires (for example two of them) which are placed in the sidewalls and experience movements correlated with those of the threads of the carcass ply. Measurement of the “thread separation” will be referred to below. It should be noted that, although this term is linked with the radial structure of a tire, the method applies not just to tires with a radial carcass. For instance, the term “thread separation” will be used to denote the average distance between two lines made on the sidewall at adjoining but different azimuths.
It should be noted that, in the event that the circumferential extension of the sidewalls is being measured within the sidewalls at a position different from their flexurally neutral fiber, the circumferential extension will include a component due to the flexure of the sidewall, in particular when passing through the contact area (a phenomenon also referred to as “bellying”). This component due to flexure is by no means a problem, and it can be exploited in order to increase the dynamic range of variation of the signals which the invention uses, by carrying out the extension measurement elsewhere than on the flexurally neutral fiber.